Gaseous or vapor-form as well as solid substances whose presence in gases or gas mixtures for electric lamps or radiators--hereinafter called gas system for brevity--is undesired and are designated as impurities. As a rule, these impurities enter the gas system in an uncontrolled way, for instance even during lamp production by means of contaminated rinsing gases or fill gases or through leaky pumping and filling systems. Even in the finished lamp, however, impurities can enter the gas system of the lamp, for instance through leaks of the lamp bulb itself or even during lamp operation, for instance through contaminated electrodes or increased electrode burnoff from inadequate fill pressure. The consequence of impurities--of whatever source--is in the final analysis nonfunctional lamps, or lamps of limited service life or maintenance.
The goal of all these efforts is therefore to assure the least possible contamination of the gas system even during lamp production. Moreover, for the sake of seamless quality control it is desirable that finished lamps with unacceptably high contamination or excessive deviations from the desired fill pressure be detected and rejected. With this background, rapid, reliable detection of corresponding impurities and monitoring of the fill pressure become quite important.
One method for determining the gas purity is known from the contribution by M. Gaugel entitled "Gasreinheitstest bei Halogen-Gluhlampen mit Hilfe eines Spektrallinienvergleichs" Gas Purity Test in Halogen Incandescent Lamps using Spectral Line Comparison! to the book series entitled "Technisch-Wissenschaftliche Abhandlungen der OSRAM-Gesellschaft" Technical-Scientific Treatises at OSRAM!, Vol. 12, pp. 546-549, Springer-Verlag, Berlin 1986. There, the light of halogen incandescent lamps is split by two beam splitters into three beams of light and delivered to three spectrally differently sensitive photomultipliers. The maximum spectral sensitivity of the individual photomultipliers is located at the wavelength of about 410 nm, 523 nm and 616 nm respectively, that is, in the blue, green and red range of the electromagnetic spectrum. By means of two analog dividers, the ratios of the "red/blue" and "red/green" photomultiplier signals are formed and compared by two comparators to find whether the corresponding values are within preadjustable tolerance range. If not, the halogen incandescent lamp is found to be defective; that is, either a fill error and/or excessive gas contamination has occurred.
A disadvantage of this embodiment is that the halogen incandescent lamp to be tested must, including its optical setup and photomultipliers, be located inside an opaque housing during the measurement. Otherwise, in-phase interference signals, such as ambient light, cause incorrect measurements, since these interference signals are weighted spectrally completely differently by the three photomultipliers and are therefore not eliminated in the quotient formation.
U.S. Pat. 4,759,630 Vuasa et al., discloses an arrangement for determining the quality of incandescent lamps. It includes a device for generating a discharge between the incandescent coil and an electrode mounted on the outer wall of the lamp bulb, and an apparatus for analyzing the radiation emitted by the discharge in the wavelength range between 550 nm and 570 nm. The lamp quality is judged from the behavior over time of this radiation, which is optionally compared with the radiation in the wavelength range between 660 nm and 680 nm.
In U.S. Pat. 3,292,988, the intensity of spectral lines of the fill gas component argon that have wavelengths greater than 650 nm is used to monitor the mercury vapor pressure in fluorescent lamps during lamp operation.
U.S. Pat. 5,168,323 proposes an apparatus and a method for determining impurities in a gas. The apparatus includes two opposed electrodes, between which the gas flows; a low-frequency alternating voltage source connected to the electrodes; and an optimal measuring instrument. With the aid of the alternating voltage source, an arc discharge is generated whose emission lines are detected by means of the measuring instrument. From the intensities of characteristic spectral lines of the various impurities, their concentration is determined with the aid of a linear equation system.